1. Field of the Invention
The present invention relates to a scheme for introducing a watermark into an information signal, such as, for example, an audio signal.
2. Description of Related Art
With the increasing spreading of the Internet, music piracy, too, has increased dramatically. Pieces of music or general audio signals are offered at many sites on the Internet to be downloaded. Only in very few cases are copyrights observed here. In particular, the author is very rarely asked for permission to make his or her work available. Even less frequently, charges as a price for legal copying are paid to the author. Additionally, works are copied in an uncontrolled manner, which in most cases also takes place without observing copyrights.
When pieces of music are legally purchased via the Internet from a provided for pieces of music, the provider will usually generate a header or a data block added to the piece of music in which copyright information, such as, for example, a customer number, is introduced, wherein the customer number unambiguously refers to the current purchaser. Also, it is known to introduce copy permission information into this header signaling most different kinds of copyrights, such as, for example, that copying the current piece is prohibited altogether, that copying the current piece is only allowed once, that copying the current piece is completely free, etc. The customer has a decoder or managing software reading in the header and, observing the actions allowed, for example only allowing a single copy and refusing further copies, or the like.
This concept for observing copyrights, however, will only work for customers acting legally. Illegal customers usually have a considerable potential of creativity for “cracking” the pieces of music provided with a header. Here, the disadvantage of the procedure described for protecting copyrights becomes obvious. Such a header can simply be removed. Alternatively, an illegal user might also modify individual entries in the header in order to convert the entry “copying prohibited” to an entry “copying completely free”. Also, it is feasible for an illegal customer to remove his own customer number from the header and then to offer the piece of music on his or her own or another homepage on the Internet. From this moment on, it is no longer possible to determine the illegal customer, since his or her customer number has been removed.
A coding method for introducing an inaudible data signal into an audio signal is known from WO 97/33391. Thus, the audio signal into which the inaudible data signal, which is referred to as watermark here, is to be introduced is transformed to the frequency domain to determine the masking threshold of the audio signal by means of a psycho-acoustic model. The data signal to be introduced into the audio signal is modulated by a pseudo-noise signal to provide a frequency-spread data signal. The frequency-spread data signal is then weighted by the psycho-acoustic masking threshold such that the energy of the frequency-spread data signal will always be below the masking threshold. Finally, the weighted data signal is superimposed on the audio signal, which is how an audio signal into which the data signal is introduced without being audible is generated. On the one hand, the data signal can be used to add author information to the audio signal, and alternatively the data signal may be used for characterizing audio signals to easily identify potential pirate copies since every sound carrier, such as, for example, in the form of a Compact Disc, is provided with an individual tag when manufactured.
Embedding a watermark in an uncompressed audio signal, wherein the audio signal is still in the time domain or in time domain representation, is also described in C. Neubauer, J. Herre: “Digital Watermarking and its Influence on Audio Quality”, 105th AES Convention, San Francisco 1998, Preprint 4823 and in DE 196 40 814.
However, audio signals are often already present as compressed audio data streams which have, for example, been subjected to processing according to one of the MPEG audio methods. If one of the above watermark embedding methods was used here to provide pieces of music with a watermark before delivering same to a customer, they would have to be decompressed completely before introducing the watermark to again obtain a sequence of time domain audio values. Due to the additional decoding before embedding the watermark, however, this means, apart from high calculating complexity, that there is the danger of tandem coding effects to occur when coding again when these audio signals provided with watermarks are coded again.
This is why schemes have been developed for embedding a watermark in audio signal already compressed or compressed audio bit streams, which, among other things, have the advantage that they require low calculating complexity since the audio bitstream to be provided with a watermark need not be decoded completely, i.e. in particular applying analysis and synthesis filter banks to the audio signal may be omitted. Further advantages of these methods which may be applied to compressed audio signals are high audio quality since quantizing noise and watermark noise can be tuned exactly to each other, high robustness since the watermark is not “weakened” by a subsequent audio coder, and allowing a suitable selection of spread-band parameters so that compatibility with PCM (pulse code modulation) watermark methods or embedding schemes operating on uncompressed audio signals can be achieved. An overview of schemes for embedding watermarks in audio signals already compressed may be found in C. Neubauer, J. Herre: “Audio Watermarking of MPEG-2 AAC Bit Streams”, 108th AES Convention, Paris 2000, Preprint 5101 and, additionally, in DE 10129239 C1.
Another improved way of introducing a watermark into audio signals refers to those schemes performing embedding while compressing an audio signal still uncompressed. Embedding schemes of this kind have, among other things, the advantage of low calculating complexity since, by pulling together watermark embedding and coding, certain operations, such as, for example, calculating the masking model and converting the audio signal to the spectral range, only have to be performed once. Further advantages include higher audio quality since quantizing noise and watermark noise can be tuned exactly to each other, high robustness since the watermark is not “weakened” by a subsequent audio coder, and the possibility of a suitable selection of the spread-band parameters to achieve compatibility with the PCM watermark method. An overview of compressed watermark embedding/coding can, for example, be found in Siebenhaar, Frank; Neubauer, Christian; Herre, Jürgen: “Combined Compression/Watermarking for Audio Signals”, in 110th AES Convention, Amsterdam, preprint 5344; C. Neubauer, R. Kulessa and J. Herre: “A Compatible Family of Bitstream Watermarking Systems for MPEG-Audio”, 110th AES Convention, Amsterdam, May 2000, Preprint 5346, and in DE 199 47 877.
In summary, watermarks for coded and uncoded audio signals in different variations are known. Using watermarks, additional data can be transferred within an audio signal in a robust and inaudible manner. Today, as has been shown above, there are different watermark embedding methods which differ in the domain of embedding, such as, for example, the time domain, the frequency domain, etc., and the type of embedding, such as, for example, quantization, erasing individual values, etc. Summarizing descriptions of existing methods may be found in M. van der Veen, F. Brukers and others: “Robust, Multi-Functional and High-Quality Audio Watermarking Technology”, 110th AES Convention, Amsterdam, May 2002, Preprint 5345; Jaap Haitsma, Michiel van der Veen, Ton Kalker and Fons Bruekers: “Audio Watermarking for Monitoring and Copy Protection”, ACM Workshop 2000, Los Angeles, and in DE 196 40 814 mentioned above.
Although the types of schemes for embedding a watermark into audio signals briefly explained before are already quite advanced, there is a disadvantage in that existing watermark methods have almost exclusively focused on the object of inaudibly embedding a watermark into the original audio signal with a high introduction rate and high robustness, i.e. having the characteristic of the watermark still being usable after signal alterations. Thus, for most fields of application the focus has been robustness. The most widespread method for providing audio signals with a watermark, i.e. spread-band modulation, as is exemplarily described in WO 97/33391 mentioned above, is said to be very robust and safe.
Due to its popularity and the fact that the principles of watermark methods based on spread-band modulation are generally known, there is the danger of methods by means of which conversely the watermarks from the audio signals provided with watermarks by these methods can be destroyed becoming known. For this reason, it is very important to develop novel high-quality methods which may serve as alternatives for spread-band modulation.